Whole human blood includes predominantly three types of specialized cells: red blood cells, white blood cells, and platelets. These cells are suspended in a complex aqueous solution of proteins and other chemicals called plasma. Although in the past blood transfusions have used whole blood, the current trend is to transfuse only those blood components required by a particular patient. This approach preserves the available blood supply and in many cases is better for the patient, since the patient is not exposed to unneeded blood components. Storage lifetimes can also be increased by packaging the individual blood products separately.
The blood components needed for transfusion may be taken from a donor by a process called apheresis in which the desired one, or more, specific components of the whole blood are separated and harvested by a blood-processing machine. The remaining components are returned to the donor. (As used herein, the term "donor" connotes anyone from whom blood is drawn for collection or processing, and can include volunteer donors, paid donors or medical patients to whom collected blood components are returned.)
Traditionally, blood components have been filtered by gravity to remove potentially deleterious contaminants or endogenous constituents, such as white blood cells or "leukocytes." Although leukocytes provide a host with protection against bacterial and viral infection, their introduction into a transfusion recipient can cause highly adverse reactions not elicited by other blood products. For example, transfused leukocytes can provoke severe immunogenic rejection reactions, viral diseases and organ damage. Accordingly, it is desirable to filter harvested blood fractions such as plasma, platelets or red blood cells to remove leukocytes.
In a typical arrangement, the blood component resides within a container in fluid communication with the inlet to a leukocyte filter and suspended thereabove; exiting filtrate is collected in a container disposed below the filter and in fluid communication with its outlet. The vertical distance between the blood component and the receiving bag is generally 20 to 60 inches.
The flow rate at which filtration occurs may vary considerably as the process proceeds. This is due primarily to gradual plugging of the filter, which increases the resistance to flow. Thus, in many cases, the flow at the onset of filtration is twice that observed at the conclusion. The filtration rate can also vary as a result of differences in blood properties among donors.
This variation can have a significant impact on filtrate quality, since filtration efficiency typically depends on contact time between the unfiltered blood component and the filter material. As the flow rate increases, for example, contact time decreases, and the concentration of unremoved leukocytes in the filtrate may therefore rise. At the same time, it is desirable to avoid needlessly low flow rates that increase the duration of the process beyond what is necessary to achieve satisfactory filtrate purity.
Generally, leukocyte filters have an appreciable internal volume, and gravity filtration is poorly suited to removing the final quantity of blood product remaining in the filter. An auxiliary fluid can, of course, be used to purge the filter and cause exit and collection of this remaining quantity, but the equipment and procedures necessary to implement this are both cumbersome and labor intensive.